Orthopedic implants include a wide variety of devices, each suited to fulfill particular medical needs. Examples of such devices are hip joint replacement devices, knee joint replacement devices, and pins, braces and plates used to set fractured bones. Particular emphasis has been recently placed on hip joint prosthetic equipment.
Contemporary orthopedic implants, including hip and knee components, use high performance metals such as cobalt-chrome and titanium alloy to achieve high strength. These materials are readily fabricated into the complex shapes typical of these devices using mature metal working techniques including casting and machining. Yet, these metals are characterized by high, fixed moduli of elasticities which makes it difficult to achieve optimal device stiffness within a given anatomical geometric envelope. In particular, in regions in which metal implants share load with surrounding bone, e.g., the medullary canal of the femur, the stress in the bone is substantially reduced versus the normal physiological level. This "stress-shielding" effect often leads to bone remodeling and may be implicated in clinical problems such as asceptic loosening and pain. Stress shielding is particularly acute in large metal implant systems. Further, large metal implants require more bone cement and are more susceptible to loosening than smaller implants.
Since metals are characterized by a single, high modulus of elasticity (16 million psi for titanium alloy and 31 million psi for cobalt-chrome alloy) it is apparent that optimal design of metallic devices must focus on the geometric part of the rigidity without regard to the material parameter. Geometric design has several constraints. For example, it is generally agreed that good bone apposition is necessary for bone ingrowth into proximal porous coatings and that close distal stem fit is necessary for rotatory stability.
Composite materials offer the potential to achieve high strength in orthopedic devices while permitting the control of stiffness for enhanced load transfer to bone. In particular, the implant designer can control modulus by varying reinforcement type, orientation and amount. Such a device is revealed in PCT patent application WO/85/04323. The device is formed from a composite material of continuous filament carbon fibers embedded within a polymer matrix. The carbon fibers in the composite material are at specific orientations relative to a specific dimension of the orthopedic device. The angularity of the carbon fibers modifies the modulus of the device. To effect fiber orientation, uniplanar sheets of carbon fibers are formed and cut into coupons. The coupons are then stacked into blocks or rolled into cylinders, to be fashioned into the final device. The manner in which the sheets or coupons are oriented will affect final mechanical properties. However, this device is limited in that the orientation of the carbon fibers cannot be systematically varied along the formed elongated body.
European Patent Publication 0277 727 discloses an orthopedic device of a biocompatible polymer with oriented fiber reinforcement. Prostheses of this reference are formed from plies of continuous filament fibers that are curvilinearly disposed within a body. The plies may have a balanced orientation; that is, for each sheet having fibers offset at a positive angle there is essentially a sheet having fibers offset at about the same negative angle. However, the prosthetic device of this variety is limited in that the orientation of the carbon fibers cannot be varied along the formed elongate body.
U.S. Pat. No. 4,750,905 reveals a prosthesis construction including an elongate, tapered polymer core containing continuous-filament fibers oriented substantially along the length of the core. The core includes an elongate distal stem. A braided sheath encases the stem. The filaments in the braid encircle the core in a helical pattern. However, devices according to this reference cannot be formed in a flexible laydown pattern as in the present invention.
It is an object of the present invention to provide an orthopedic implant with variable modulus wherein the stresses in the surrounding bone are more nearly equal to their normal physiological level than achieved in a system without modulus variations. It is a feature of the present invention to provide a variety of composite materials to design an orthopedic implant with particular properties. It is an advantage of the present invention that the subject orthopedic implants have a variable modulus along their lengths due to the use of filament winding and braiding techniques.
These and other objects, features and advantages of the present invention will become more readily apparent with reference to the following description of the invention.